It is previously known to manufacture silica aerogel in pulverulent form. However, as far as can be ascertained, it is not known to prepare silica aerogel in the form of large blocks, and, above all, not substantially crack-free such blocks.
By way of the example of the prior art, mention might be made of U.S. Pat. No. 3,672,833 which describes the preparation of silica aerogel by hydrolysis of lower alkyl orthosilicate in an alcohol, and heating the formed silica gel in an autoclave together with further alcohol to a temperature above the critical point of the alcohol. The addition of further alcohol is made in order that the critical pressure be exceeded when the temperature is raised above the critical point. Thereafter, the pressure in the autoclave is reduced in that water and alcohol vapor are released, the autoclave is flushed out with dry inert gas and is reclosed, and is then cooled to room temperature. Those conditions which are utilized according to the U.S. patent result in the obtention of the silica aerogel in the form of a granular material which is intended to be used as, for example, gelification or thickening agents.
According to the U.S. patent, the hydrolysis of the alkyl orthosilicate may be carried out in acid, neutral or basic media, acid hydrolysis with acetic acid being preferred. In hydrolysis in a basic medium, a colourless and transparent aerogel is obtained.
There is a need in this art to be able to prepare such silica aerogel in a form of large, whole and crack-free blocks, in particular transparent such blocks.
Such silica aerogel has, namely, exceptionally low thermal conductivity with a consequently high level of effectivity as a thermal insulation material, good resistance to high temperatures, high adsorption capacity for gases, and is, moreover, preferably transparent. In particular this latter form which is transparent, that is to say allows the passage of radiation within the visible range of the spectrum, is of particular interest as windows in planar solar panels. A solar panel which is provided with such silica aerogel readily allows the passage of solar radiation into the solar panel but prevents the heat which is thus generated in the solar panel from being dissipated. Furthermore, the transparent material is usable as window insulation, in that a layer of the material is introduced between two window panes in order to improve thermal insulation without appreciably obstructing view through the window.